This invention concerns a continuous-casting crystalliser with increased heat exchange and also a method to increase the heat exchange in a continuous-casting crystalliser.
The invention is employed in association with a mold used in a continuous casting plant for the production of billets, blooms or slabs of any desired type and section.
The field of continuous casting still entails a plurality of problems which have not yet been overcome and which are linked to the high temperatures to which the sidewalls of the crystalliser are subjected.
To be more exact, it is known that the temperature of the sidewalls of the crystalliser, notwithstanding the circulation of cooling fluid, changes in the direction of the casting with a maximum value reached in the vicinity of the meniscus of the molten metal.
The uneven temperature along the sidewalls of the crystalliser causes an uneven deformation of those sidewalls together with their outward displacement in relation to their initial position in the cold state, this deformation being due to the thermal expansion of the material, with resulting problems linked to the surface faults caused by this uneven deformation on the billets/blooms/slabs being formed.
Moreover, it is known that the skin of the solidifying billets/blooms/slabs during their descent in the crystalliser shrinks according to a law which differs from one material to another.
The combination of these two factors causes, at least in the lower zone of the crystalliser, a separation of the skin of the billet/bloom/slab from the sidewalls of the crystalliser and reduces considerably the heat exchange between the billet/bloom/slab and the crystalliser to the extent that the cooling and therefore the formation of the skin are practically halted with very severe results for the billet/bloom/slab being formed.
In the crystallisers of the state of the art the coefficient of heat exchange between the forming skin and the sidewalls of the crystalliser, at least in the lower zone of the crystalliser, takes on values which are lower than 36000 W/m.sup.2 K and which are therefore not acceptable for an efficient action of cooling and therefore of solidification of the skin being formed.
The article of J. K. BRIMACOMBE "Empowerment with Knowledge--Towards the Intelligent Mould for the Continuous Casting of Steel Billets", METALLURGICAL TRANSACTIONS B, Volume 24B, DECEMBER 1993, pages 917-930, shows clearly that in crystallisers of the state of the art the heat flux in the zone of the exit of the cast product from the crystalliser is between about 1.2 and 1.4 MW/m.sup.2, whereas it does not exceed 2 MW/m.sup.2 in the zone where the separation of the skin from the sidewalls of the crystalliser begins.
In the crystallisers of the state of the art, therefore, the heat exchange has acceptable values only along the first segment of the crystalliser, which extends along about a quarter of the length of a crystalliser and normally about 200 mm. below the meniscus; in this first segment the skin of the billet/bloom/slab is substantially in contact with the sidewalls of the crystalliser.
So as to ensure that the billet/bloom/slab leaving the crystalliser has a thickness of skin such as to prevent its breakage and the resulting break-out of liquid metal, it is therefore necessary to employ a reduced casting speed.
Where the billets or blooms have a square, rectangular or generally polygonal cross-section, another problem is linked to the fact that the corners of the billet or bloom undergo a more intense cooling since at those corners the heat is removed on both sides of the corner.
The result is that at the corners of the billet or bloom the skin forms more quickly and the resulting shrinkage of the material has the effect that the billet or bloom is separated very soon from the sidewalls of the crystalliser, thus interrupting the cooling and solidifying process.
For this reason the skin of the billet or bloom at the corners is less thick than along the sidewalls of the billet or bloom, and gradients of temperature between the corners and the sides of the billet or bloom are created.
These temperature gradients generate tensions both within the sidewalls of the crystalliser and within the billet or bloom being cooled, and these tensions lead to the formation of cracks and other surface faults which reduce the quality of the outgoing product.